Boost converters are used to provide power factor correction and to increase the magnitude of a DC voltage at a power output relative to a DC voltage at a power input. Typically the voltages at both the input and output are not closely regulated and may vary considerably in instantaneous magnitude. However, these input and output voltages have a substantial average DC component and it is common to refer to them as DC voltage signals.
A boost converter operates by connecting an input voltage through an inductor and then a diode to the output. Filter capacitors are usually provided between both the input and common and the output and common to stabilize and hold the DC voltages at both locations. A power switch, which is typically a MOSFET, is connected to shunt current from the junction of the inductor and diode to common, which may be neutral or ground. When the switch is closed the inductor begins conducting current to common and stores energy as the current magnitude increases. When the switch is opened, the stored energy of the inductor provides current to the converter output at a voltage greater than the input voltage. A control circuit controls the duty cycle or percentage of time during which the switch is closed in a feedback loop that is responsive to the output voltage to maintain the output voltage at a desired magnitude.
Boost converters have numerous applications in switching power supplies, but exhibit reduced efficiencies because of power losses that occur in the boost converters. A large source of power loss is the conversion of electrical energy to heat energy during the switching transient as the switch changes between conducting and nonconducting states.
Boost converters may be divided into two categories according to their operating characteristics. A continuous conduction converter maintains a continuous non-zero current in the inductor. Before the inductor current falls to zero the switch is turned on to increase the energy stored by the inductor and the current therethrough. It is usually found more cost effective to use a continuous conduction converter at power levels above 250 watts. The continuous conduction converter experiences less steady state conduction loss because the RMS current in the switch is lower when the current does not fall to zero during each cycle. However, the continuous conduction converter experiences significant transient power losses at both turn-on and turn-off of the power switch. At turn on the switch has the full output voltage across it until the switch current rises to the magnitude of the inductor current, while during turn-off the voltage across the switch rises to the level of the output voltage much faster than the current drops to zero. Since transient power losses in the switch are proportional to the product of instantaneous voltage and instantaneous current, the simultaneous occurrence of substantial current through the switch and substantial voltage across the switch results in significant power losses.
In contrast, a discontinuous conduction converter does not close the switch until after the diode current has dropped to zero. A discontinuous conduction converter is usually more cost effective in applications below 250 watts, has a higher steady state conduction loss and has a substantial transient turn off loss. However, the turn on loss can be reduced by turning on the power switch under near zero current, near zero voltage conditions that minimize the turn on power loss. Although it is known to use a discontinuous boost converter under conditions that reduce the power loss therein compared to a continuous converter, conventional discontinuous boost converters still experience substantial power losses and a need exists to reduce these power losses even further.